The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for forming copper wiring.
As the increasing miniaturization and higher performance of silicon semiconductor products are pursued in recent years, copper (Cu) wiring has been used on more frequent occasions. Since the dry etching of copper is extremely difficult, the formation of Cu wiring is performed typically by a sequence of process steps (damascene method) including a dry etching step performed with respect to an inter-wire insulating layer to form wire grooves, an electrolytic plating step for burying copper in the formed wiring grooves, and a chemical mechanical polishing (CMP) step for removing the excess portion of a Cu film and flattening the Cu film.
On the surface of a semiconductor substrate immediately after the CMP step, particles of a slurry and polished Cu dust and metal remain in a large quantity so that the cleaning of the substrate is performed to remove the residues. After the polishing, Cu wires are exposed at the surface of the substrate so that, if the removal of the residues is insufficient, it causes degraded wire performance and, especially, a short circuit between the wires. This presents a large problem to the performance of a semiconductor device.
For the cleaning of the semiconductor substrate after the CMP step, RCA cleaning has been used widely. A typical RCA cleaning sequence consists of the step of removing particles by using an alkaline solution (a solution mixture of ammonia and an aqueous hydrogen peroxide: APM), the step of removing an oxide film by using a dilute hydrofluoric acid, and the step of removing metal contamination by using an acid solution (a solution mixture of a hydrochloric acid and an aqueous hydrogen peroxide: HPM). If the RCA cleaning is performed with respect to Cu wires, however, the problem is encountered that the Cu wires are prone to etching because ammonia forms an ammine complex with Cu and the hydrofluoric acid roughens the surfaces of the Cu wires.
For a cleaning process after the formation of Cu wires, therefore, a method using a combination of mechanical cleaning employing a brush and chemical cleaning employing an organic acid (an oxalic acid) has been implemented normally. This is a method which removes particles by the mechanical effect of the brush and removes Cu contamination by forming a chelate complex of an oxalic acid with a copper oxide. In accordance with the cleaning method, however, it is difficult to remove a wire-to-wire bridge formed between Cu wires by polishing, though it is possible to remove extremely fine residues including particles and metal contamination to a degree. The mechanism of the occurrence of the wire-to-wire bridge in question will be described herein below with reference to FIG. 8 showing the geometries of the wire-to-wire bridges.
In the CMP step for a Cu film, the Cu film and a barrier film are polished and the polishing is ended when an inter-wire insulating layer 2 is exposed. To prevent an increase in wiring resistance resulting from a reduction in the film thickness of each of the Cu wires 6, polishing is performed such that the upper surface of the Cu wire 6 has a configuration protruding from the upper surface of the inter-wire insulating layer 2.
When pressure is exerted in a concentrated manner on the Cu wire 6 having such a protruding configuration during the polishing, the Cu wire 6 is crushed under the pressure to expand over the inter-wire insulating layer 2 so that a wire-to-wire bridge 9 is formed to provide conduction between the adjacent Cu wires 6, as shown in FIG. 8A.
When the Cu surface is shaved by a foreign substance during polishing and a scratch 21 is formed, the shaved Cu expands over the inter-wire insulating layer 2 so that a wire-to-wire bridge 9 as shown in FIG. 8B is formed. With polished Cu dust being buried in the scratched groove, a wire-to-wire bridge 9 as shown in FIG. 8C is formed.
The wire-to-wire bridge thus formed should be removed since it causes a short circuit between the wires and a reduction in the yield of a semiconductor product. In accordance with the conventional cleaning process using the brush and the oxalic acid, however, only extremely fine particles and metal contamination can be removed and such a wire-to-wire bridge cannot be removed.
On the other hand, there has been a cleaning process using ozonated water and a hydrofluoric acid as a cleaning method replacing the conventional RCA cleaning and the application of the cleaning process using ozonated water and a hydrofluoric acid to a Cu wiring step is proposed in, e.g., Japanese Laid-Open Patent Publication No. HEI 8-153698.
The cleaning method allows the removal of residues such as particles and metal on the substrate surface therefrom by oxidizing the Cu surface by using the ozonated water and removing a resulting oxidation layer by using the hydrofluoric acid. The cleaning method also allows even a wire-to-wire bridge composed of a Cu film to be removed to a degree.
As a result of closely examining the substrate surface after the process using the ozonated water and the hydrofluoric acid was performed by using an optical pattern defect inspection system, however, it was proved that 1000 or more depressed defects ranging in diameter from 100 nm to 300 nm were formed in the surfaces of Cu wires over the entire substrate surface.
FIG. 9 diagrammatically shows an outward appearance of a depressed defect 22 that has been observed. It can be considered that, since the oxidation-reduction potential of the ozonated water is as high as 1.1 V, Cu on the surface of each of the Cu wires 6 is ionized by the resulting battery effect and dissolved in the ozonated water, thereby causing such a depressed defect 22. When a large number of depressed defects 22 are formed in the surfaces of the Cu wires 6, there is the possibility that the wiring resistance increases to further cause a broken wire. As a result, the problem is encountered that it is extremely difficult to apply the cleaning process using the ozonated water and the hydrofluoric acid to a Cu wire forming step.
As a result of evaluating a wire-to-wire bridge removal rate achieved by the cleaning process using the ozonated water and the hydrofluoric acid by using an electronic pattern defect inspection system, it was also proved that the cleaning process using the ozonated water and the hydrofluoric acid only allowed the removal of about 75% of the wire-to-wire bridges.